Electromagnetic field exposure is reported to decrease circulating melatonin levels. Preliminary studies have suggested that melatonin can reduce cell growth. Melatonin (10-9M) attenuated proliferation in BG1 and MCF7 cells. The goal of this project is to investigate the signaling pathways by which melatonin inhibits proliferation. Since there are several reports suggesting that EMF exposure can alter cell calcium homeostasis, we first investigated whether melatonin altered cell calcium. We have found that acute addition of melatonin does not alter ionized cell calcium (Cai). BG1 cells were loaded with fura2-AM and Cai was monitored using fluorescence microscopy in cells loaded with the fluorescent calcium indicator, fura-2. Addition of melatonin (10-5 to 10- 9M) did not alter Cai. This lack of effect on Cai has lead us to investigate other signaling pathways. We are particularly interested in determining whether melatonin is acting via a receptor mediated or non- specific pathways (e.g. as an antioxidant).We are currently investigating whether the oncostatic properties of melatonin can be blocked by receptor antagonists. Melatonin has been reported to bind to plasma membrane receptors (Trends in Pharmacol. Sci. 16: 50, 1995). Melatonin is also reported to bind to an orphan receptor of the nuclear receptor superfamily and cause repression of the 5-lipoxygenase gene (J. Biol. Chem. 270:7037, 1995). We are examining the involvement of both plasma membrane receptors (antagonized by luzindole) and nuclear receptors (antagonized by CGP52608). We are also investigating whether melatonin acutely alters intracellular mediators such as pH, cAMP, diacylglyceride or ceramide.